288 



RESPIRATION. 



less oxjrgen than would be required to convert all its hydrogen into 

 water. 



It is no doubt for these reasons that, in herbivorous animals, feeding 

 largely on the carbohydrates, the quantity of oxygen exhaled in the 

 carbonic acid should be nearly equal to that taken in with the breath ; 

 while in the carnivora, which consume only fats and albuminous matters, 

 a larger proportion of oxygen should disappear from the products of 

 respiration. 



Finally, some kinds of vegetable food, as fruits and green tissues, 

 contain certain substances in which the oxygen is more than sufficient 

 to form water with the hydrogen present. Such are the salts of vegeta- 

 ble acids, like oxalic, citric, gallic, malic, and tartaric acid. The result 

 of the internal consumption of tartaric acid, for example, would be as 

 follows 



Tartaric acid. 



Carbonic acid. 



= c,o 8 4- 



Water. 



H 6 3 . 



In this instance more oxygen will be exhaled, in the carbonic acid 

 produced, than was absorbed from the atmosphere; because a super- 

 abundance already existed in the material used as food. 



The relative proportions of oxygen and carbonic acid, absorbed and 

 expired in respiration, will therefore vary, as has been well shown by 

 Mayer, 1 not only with the nature of the food, but also according to the 

 transformations, in the interior of the living organism, of one nutritive 

 substance into another, as of a carbohydrate into a fat, or of either into 

 an organic acid. In the fermentation of a saccharine solution there is 

 even an elimination of carbonic acid without the absorption of any 

 oxygen whatever ; this process being one, not of direct oxidation, but 

 of the rearrangement of the elements already present in the sugar, a 

 portion of them being exhaled as carbonic acid, while the rest remain 

 behind in the form of alcohol. 



In the animal body the function of respiration consists, first in the 

 absorption of oxygen, and secondly in the exhalation of carbonic acid. 

 It is evidently, therefore, so far as its consequences are concerned, an 

 act of oxidation. But the elements of the food are in no case subjected 

 to immediate oxidation. They are digested in the alimentary canal and 

 taken up into the circulating fluid under other forms of organic com- 

 bination. These undergo still further transformations, both in the blood 

 and in the tissues, passing through a series of successive metamorphoses, 

 until they finally leave the body, principally under the forms of urea, 

 carbonic acid, and water. Oxidation, accordingly ? as it occurs in the 

 living body, is not so much the immediate process as the result of the 

 vital operations, and is very different from the direct combustion of 

 hydrocarbonaceous matters in the atmosphere. 



Exhalation of Watery Vapor in Respiration. The watery vapor, 

 exhaled with the breath, is given off by the pulmonary mucous mem- 



1 Lehrbuch der Agrikultur-Chemie. Heidelberg, 1871, p. 101. 



